1. Field of the Invention
The present invention relates to an electrostatic discharge (ESD) protection device and a method for producing the same. In particular, the present invention relates to an improvement in a discharge auxiliary electrode provided for the purpose of accelerating electrostatic discharge in an ESD protection device.
2. Description of the Related Art
An overvoltage protection element that is interesting to the present invention is described in, for example, Japanese Unexamined Patent Application Publication No. 2008-85284 (Patent Document 1).
Patent Document 1 describes an overvoltage protection element material which is to become a discharge auxiliary electrode provided for the purpose of accelerating discharge. The material contains a non-conductor powder (for example, silicon carbide: particle size 1 to 50 μm) and a metal conductor powder (for example, copper: particle size 0.01 to 5 μm), and an adhesive (for example, glass powder).
In addition, Patent Document 1 describes a method for producing an overvoltage protection element, the method including a step of preparing a material paste by uniformly mixing a non-conductor powder, a metal conductor powder, and an adhesive at a particular ratio, a step of printing the material paste on a substrate, and a step of conducting a firing process (temperature: 300° C. to 1,200° C.) on the substrate.
However, there is a limitation in enhancing discharge characteristics in the overvoltage protection element described in Patent Document 1. The reason for this is as follows. In the overvoltage protection element described in Patent Document 1, in order to enhance discharge characteristics, the content ratio of the metal conductor powder to the non-conductor powder may be increased. However, in the case where the content ratio of the metal conductor powder is increased, the metal conductor powder is in a state in which the surface thereof is exposed. Consequently, the exposed metal conductor particles are bound to each other during discharge, which may decrease insulation reliability. Silicon carbide, which is used as a non-conductor powder in the overvoltage protection element described in Patent Document 1, is a semiconductor having a relatively low insulation resistance, and thus silicon carbide does not easily contribute to the improvement in insulation reliability.
For example, International Publication No. 2009/098944 pamphlet (Patent Document 2) describes a technology that can solve the problems described above.
Patent Document 2 describes the use of a discharge auxiliary electrode in which a conductive material (such as a Cu powder) coated with an inorganic material (such as Al2O3) is dispersed. According to the technology described in Patent Document 2, since the exposure of the conductive material is less than that in the technology described in Patent Document 1, insulation reliability can be enhanced. Furthermore, even when the content of the conductive material is increased, short-circuit between the particles of the conductive material does not easily occur. Thus, discharge occurs more easily by increasing the content of the conductive material. Consequently, the discharge characteristics can be improved.
However, regarding the technology described in Patent Document 1, in order to improve discharge characteristics, there is no solution but to increase the content of the conductive material. Therefore, the improvement in discharge characteristics is limited. Accordingly, it is desirable to find a method for further improving discharge characteristics.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-85284    Patent Document 2: International Publication No. 2009/098944 pamphlet